神经错乱 is a four-character set phrase (成语) that literally means 'nerve disordered'. It describes a state of mental disturbance, madness, or irrational behavior — stronger and more colloquial than 精神病 ('mental illness' in a clinical sense). You can use it as a noun ('insanity') or as a predicate ('to go crazy'). It often carries a dramatic or judgmental tone, so be careful in formal writing.